Research Progress of Large-scale Integrated Optical Microcavities for Sensing(Invited)
Sensors have been widely applied in various fields,such as environmental monitoring,biomedical engineering,and food safety.However,in recent years,there has been a growing demand for miniaturized sensing equipment or systems that can be used in portable or wearable applications.These application scenarios have imposed new requirements on sensing device size,weight,power consumption,and integration levels.Highly integrated sensing systems primarily rely on measuring changes in electrical or optical parameters.Compared with electrical sensors,optical sensors possess the advantages of immunity to external electromagnetic interference,stability,and label-free sensing capability.Fiber optic sensors are the most mature optical sensing method,known for their high sensitivity and low cost.However,they are constrained by their relatively large device size and the challenges associated with micro/nanofabrication,making it difficult to achieve high spatial density sensing arrays.Similarly,Surface Plasmon Resonance(SPR)sensors offer high sensitivity but require spatial optical coupling,resulting in a lower level of integration as sensing equipment.However,a compact integrated optical sensing device provides numerous benefits.It combines the advantages of optical sensing with low processing costs and high integration capabilities due to its compatibility with CMOS-compatible processes.Furthermore,by cascading multiple integrated photonic devices in an array,multi-site and multi-parameter sensing capability can be achieved with exceptional spatial resolution on a large scale,significantly enhancing the efficiency and functionality of the sensing system.Based on the method of light coupling to the optical sensors,the architecture for realizing multi-channel sensing arrays can be categorized into three types:mechanical scanning,spectroscopic scanning,and wavelength division multiplexing.Mechanical scanning approaches necessitate large dimensions of sensing systems and result in low operational efficiency.Spectroscopic methods reduce the optical power of single-channel sensing signals,with the number of channels limited by detector signal-to-noise ratio.Wavelength division multiplexing techniques exhibit high spectral utilization and are suitable for large-scale sensing applications,with the quality of sensing signals unaffected.However,sensing throughput is mainly limited by the tunable laser tuning range or the bandwidth of on-chip spectrometers.Integrated optical microcavities,typically smaller than conventional MZI and long waveguide structures,offer high-sensitivity responses due to their narrow line widths and could be cascaded for broadband operation,making them ideal for array sensing and enhancing sensing throughput.Optical microcavity structures serving as sensors mainly include micro-ring and micro-disk structures based on whispering gallery modes and defect and F-P type photonic crystal structures.Optical resonance peak shifts,splitting,and broadening of microcavities based on whispering gallery modes have been achieved in numerous studies of optical sensing at the single-molecule and single-particle levels.However,these studies rely on microcavity structures requiring three-dimensional fabrication techniques,making mass production challenging.Micro-disks can excite multiple modes,making it difficult to track resonance wavelengths.Micro-ring fabrication is relatively easier with high spectral utilization,making it the most commonly used integrated optical sensing unit.However,the periodicity of micro-ring spectra imposes limitations on the number of cascaded units that can be used,as the free spectral range constrains it.Defect photonic crystal structures are small in size with small mode volumes and low process tolerance,but the quality factor of unoptimized devices is limited.F-P type nanocavity microcavities are easier to fabricate but exhibit less optical field leakage and weaker interaction between light and matter,resulting in lower sensitivity.One-dimensional grating microcavity structures based on F-P cavities exhibit high sensitivity due to increased optical field leakage,but introduce more scattering losses due to additional sidewall roughness from fabrication,similarly limiting the quality factor.Although these three types of photonic crystal structures can achieve high throughput multi-channel cascading by coupling on the waveguide side,most devices perform worse than conventional micro-rings.Additionally,the high-precision micro/nano fabrication process required for these devices is beyond the capabilities of many foundries.While recent research has made significant progress in micro/nanofabrication and heterogeneous integration technologies,most efforts have focused on integrating sensing units with detection,leaving the research on fully integrated sensing systems relatively limited.Large-scale sensing arrays are now capable of mass production.However,challenges remain in realizing miniaturized high-throughput sensing systems and promoting industrial development,including insufficient scalability of sensing units,low integration of sensing systems,and a lack of mature packaging methods and testing processes.It is expected that in the future,more researchers will jointly promote the development of optical sensing systems through the optimization of optical structures and fabrication processes,drawing from existing large-scale system designs to improve performance,reduce dependence on high-precision,high-cost sensing unit design and fabrication,as well as reliable system packaging and testing processes.
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